Turbidity is the lessening of the transparency of a medium, for example, a liquid, caused by the presence of undissolved substances, since light radiated into the medium is scattered on these undissolved substances. In the case of optical turbidity sensors, the turbidity value of a liquid is determined by measuring this scattered light.
Turbidity measurements in the sense of this invention are performed by means of a turbidity sensor, especially in fresh and industrial water as well as in gases. Furthermore, the invention is concerned with measurements of similar process variables such as solids content or sludge level. Measuring devices suitable for determining the corresponding process variables are manufactured and sold by the group of firms, Endress+Hauser, in a large number of variants, for example, under the designation “Turbimax CUS51D”.
Usually, the sensors are arranged in housings, and the determining of the process variable occurs, such as already mentioned, optically. In such case, electromagnetic waves of a certain wavelength are sent from at least one light source, scattered by the medium being measured and the scattered waves received by a light receiver. In such case, “light” in the sense of this invention is not limited to the visible region of the electromagnetic spectrum, but, instead, can be electromagnetic radiation of any wavelength, especially also radiation in the far ultraviolet (UV) and in the infrared (IR) wavelength ranges. For example, the wavelengths of the electromagnetic waves of the optical components lie typically in the near infrared, for example, at 860 nm.
Applied as light sources are, most often, narrow band radiators, e.g. a light emitting diode (LED). In such case, the LED is used for producing light lying in a suitable wavelength range. Correspondingly applied as light receiver can be a photodiode, which produces from the received light a receiver signal, for example, a photocurrent or a photovoltage.
The sensors are installed in containments. Light scatterings or reflections on the walls of the containments corrupt the measurement signal, when such are detected by the sensor. Since the influence of these wall effects increases with declining turbidity of the medium (smaller extinction by the medium and smaller measurement signal), in the case of small clouding of the measured medium (e.g. in the clear water or drinking water domains), wall effects make turbidity measurement almost impossible.
Therefore, in practice, an effort is made to position optical sensors with as large separation as possible from walls, for example, using large volume containers.
From the point of view of the user, it is, however, often advantageous to position the sensors directly in a pipeline, respectively process line. The optical elements are, as a rule, positioned externally on the straight, right angled end face of the sensor and the transmitted light is radiated inclined into the medium. Since the sensor in the case of this tube installation is surrounded peripherally by surfaces, different disturbances result from wall effects as a function of the pipe diameter.
As a rule, it is indicated in operating instructions that the sensors should be positioned as far as possible from walls or from the floor or so to orient them that the transmitted light is not directed at wall areas or floors. It is, thus, frequently recommended that the sensor be installed inclined, i.e. at an angle relative to the pipe or containment. This positioning, for instance, at an angle of 45°, is, however, undesirable in practice, since pipe fittings, flanges etc., are, as a rule, embodied at an angle of 90° to the pipe axis.
Therefore, sensors frequently have to be readjusted in the installed state, in order to match them to the particular situation. This can occur, for instance, by the adjusting of one or more measurement points. The measurement signal corrupted by wall effects is, in such case, associated with a reference value. Another approach is to adjust the measured value as a function of the installed situation via experimentally ascertained correction factors. In other cases, sensors are coordinated with a pipe piece or a flow through cell in the plant and these delivered as matched “pairs”. In all described examples, there results for the user a significant effort, which does not solve the actual problem, but, instead, at most, ameliorates it.